Numerous muffler constructions have been proposed for the attenuation of the sound component of an exhaust gas stream from an internal combustion engine. Invariably, these structures have purported to effect sound attenuation without substantially or intolerably increasing the back pressure on the engine. As is well known, muffler induced back pressure will substantially reduce engine performance.
The problem of reduced performance is most extreme in high-performance racing engines. The "solution" to the problem which is actually used in the racing industry usually is to employ a straight pipe from the engine and tolerate the noise. With urban expansion, however, even race tracks are under pressure to reduce the noise level during racing. Moreover, at least some high performance cars also are driven, at least occasionally, on the city streets. In order to be "street-legal" such high performance engines must be coupled to a muffler, and the only mufflers which are currently commercially available that are used on such high-performance engines cause a significant drop in engine power as a direct result of the back pressure induced in the muffler.
Typically, a 575 horsepower engine will produce a noise level of about 130 decibels (db) at hard acceleration with no muffler, and on the same engine when a commercially available high-performance muffler is used, the noise level will be reduced to about 95 db (A scale) at hard acceleration, but there also will be an 18% to 20% power loss. Even larger engines, for example 700 to 800 horsepower, have more cam overlap and cannot tolerate sound attenuation to 95 db since it would produce a 30% to 40% power loss.
Another problem that complicates any attempt to attenuate sound in high-performance internal combustion engines is the necessity to minimize bulk and weight. The exhaust pipe on a high horsepower engine typically will be about 4 inches in diameter so as to accommodate the very substantial volumetric flow. Mufflers which depend upon excessive length or diameter to achieve sound attenuation will be unsuitable for use on race cars, either because of their bulk or weight, or both.
The patent art contains various muffler constructions which purport to solve the problem of sound attenuation without undesirable back pressure, but in fact these various structures have substantial performance deficiencies. It is well known to provide a divergently tapered centrally located conical partition for flow of gases around the partition to effect an expansion of the gases. Typical of such structures are the devices shown in U.S. Pat. Nos. 2,071,351; 2,239,549; and 2,971,599. Some of these patented mufflers follow such an expansion partition or cone with a contraction or concentrating partition or baffle. Typical of such devices are the mufflers shown in U.S. Pat. Nos. 1,081,348; 2,667,940; 3,029,895; and 3,29,896. These mufflers, however, do significantly increase back pressure by causing the exhaust gases to reverse the direction of their flow axially a they attempt to pass beyond the concentrating or converging baffle. This flow reversal may be effective in sound attenuation, but it has been found to increase back pressure undesirably.
Even mufflers which employ alternating divergent and then convergent partitions have suffered from undesirable bulk and/or weight, inordinate complexity, or auxiliary flow channels or openings in the partitions which defeat sound attenuation. Typical of such mufflers are the mufflers set forth in U.S. Pat. Nos. 624,062; 1,184,431; 2,325,905; and 2,485,555.
Additional patent art known to applicant but believed to be peripheral in relevance to the present invention are the following U.S. Pat. Nos. 1,677,570; 1,756,916; 1,946,908; 2,934,889; 3,219,141; 3,786,896; 4,143,739; and 4,346,783.
The reality of the industry is that high-performance racing cars are either using no muffler or mufflers which barely achieve the desired sound attenuation, and achieve it at a significant power loss and with an undesirable increase in bulk and weight.
An additional complication results when a high-performance or conventional internal combustion engine is turbocharged. The exhaust gases from such turbocharged engines exit the engine in a rather turbulent stream, instead of coherent pulses typical of engines which are not turbocharged. Thus, the effect of turbo charging on the exhaust gases from an internal combustion engine is to substantially increase the turbulence of the gases as they enter the muffler.
In a turbocharged engine the turbulence also tends to entrain the sound in a more uniform manner throughout the volume of the exhaust gases as compared to an unturbocharged engine in which the sound can be preferentially distributed in the pulses. In the unturbocharged engine, therefore, a back pressure increase can enhance the uniformity of sound attenuation by the muffler partition system, but for turbocharged exhausts any back pressure increase in the muffler is simply undesirable because the sound component is already thoroughly mixed with the volume of the exhaust gases.